warn users about upgrading to pg15 when using borg

docs/maintenance-postgres.md

@@ -76,6 +76,8 @@ This is because newer Postgres versions cannot start with data generated by olde
 
 Upgrades must be performed manually.
 
+**Warning: If you're using Borg Backup you probably don't want to upgrade to Postgres 15 yet as there is currently no support.**
+
 This playbook can upgrade your existing Postgres setup with the following command:
 
 	ansible-playbook -i inventory/hosts setup.yml --tags=upgrade-postgres